Intermittent Fasting, Cutting Calories Give Same Weight Loss

Eight-hour intermittent fasting had similar weight-loss results to calorie counting in adults with obesity, a new study published online in the Annals of Internal Medicine has found. 

The small, unblinded study compared weight loss in 77 participants who either intermittently fasted, adhered to a calorie-restricted diet, or were in a control group with no eating restrictions.

Compared with the control group, absolute weight loss for people in the intermittent fasting group was about 4.6 kg (10 lb) compared with 5.4 kg (12 lb) for those in the calorie-restriction group, after 12 months, with no significant difference between the intervention groups. 

Intermittent fasting, or time-restricted eating, relies on the idea that the time you eat is more important for weight loss than what or how much you eat. The term is a catch-all for eating patterns that could include several full days of fasting per week or time-restricted eating during the day. 

The effect of having less time to eat is thought to lead to the consumption of fewer calories, thought to be the main reason the approach works. Indeed, this trial found the intermittent fasting group ate 425 fewer calories per day, compared with 405 fewer calories per day in the calorie-restricted group. 

“Time-restricted eating is undoubtedly an attractive approach to weight loss in that it does not require the purchase of expensive food products, allows persons to continue consuming familiar foods, and omits complicated calorie tracking,” Shuhao Lin, RD, University of Illinois Chicago, and colleagues write.

During the trial, participants were in a weight-loss phase for 6 months. The intermittent fasting group could eat anything they wanted to between 12 PM and 8 PM, and didn’t have to count calories. The later time window is on par with the eating pattern of most people in the United States who fast.

The calorie-restriction group had to cut 25% of their daily calorie intake based on their total energy expenditure. They were also told to fill half of every plate with fruits or vegetables, and consume about half their energy as carbohydrates, 30% as fat, and 20% as protein. 

The weight-loss phase was followed by a 6-month weight-maintenance phase. During this phase, the window for eating was extended from 10 AM to 8 PM for the intermittent fasting group, and the calorie-restriction group was told to match their energy needs, which overall, had reduced by about 15% compared with baseline. 

Most participants were women with a mean body weight of about 100 kg (220 pounds) at baseline. 

Both the time-restricted eating and calorie-restriction groups regularly met with dieticians, which the authors of an accompanying editorial say could have made the intermittent fasting more effective than in previous trials.  

An earlier, shorter trial found about 0.9 kg (2 lb) weight loss after 12 weeks of adhering to a similar eating window, a more modest result compared with the 4 kg (9 lb) weight loss at 6 months in this trial.

“The difference in outcomes between these two trials is likely attributable to differences in dietary counseling,” write the editorialists, Adam Gilden, MD, and Victoria Catenacci, MD, from University of Colorado Medicine, Aurora.

Previous studies of intermittent fasting have been short and showed similar findings compared with a calorie-restricted diet.

The study was funded by the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health.

Ann Intern Med. Published online June 26, 2023. Abstract, Editorial

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